This invention relates to ultrasonic inspection of materials. More particularly, this invention relates to calibration standards employed with ultrasonic testing systems.
Ultrasonic inspection is widely used for the non-destructive testing of many types of materials, such as pipes, pressure vessels, welds, conduits and the like. In a typical ultrasonic testing procedure, a transducer operable in the ultrasonic range is placed on a surface of the component being tested and repeatedly energized to create pulsed ultrasonic waves which are coupled to the interior of the component. The waves travel through the interior of the component, and are reflected at surfaces or interior discontinuities and subsequently detected by either the same transducer operated in a receiving mode or a second transducer at the same location as the generating transducer or a different location spaced therefrom. By noting the arrival time and intensity of the received sound waves, cracks, fissures, or other inhomogenieties in the interior of the component can be detected.
In order to correctly interpret the received waves, the inspection apparatus must be calibrated by means of a calibration standard. A typical calibration standard is constructed from the same or a similar material to that from which the components to be tested are fabricated, and is provided with a controlled geometry reflector, such as a cylindrical hole of precise dimensions. In use, the transducer of the inspection system to be calibrated is placed on a surface of the calibration standard at a known distance from the calibration reflector and energized to generate pulsed ultrasonic waves in the interior of the calibration standard. By measuring the time interval between pulse generation and reception of the reflected wave, the speed of the waves in the particular material can be determined. This information enables the distance between the transducer and a reflector surface in a component under inspection to be determined from the time interval between pulse generation and reception. In addition, by measuring the attenuation of the wave in traveling through the calibration standard from the sending transducer to the reflector and to the receiving transducer, a calibration scale showing the attenuation of the ultrasonic waves with distance in the material, commonly termed "a distance amplitude correction scale" or DAC, can be plotted.
While existing calibration standards of the above type have been found to be adequate for components having planar surfaces, in applications requiring the ultrasonic inspection of components having curved surfaces such standards have been found to lead to inaccurate results. In known calibration standards used with curved surfaces, the calibration hole containing the controlled geometry reflector surfaces is a cylindrical bore passing through the body of the calibration standard. Due to the internal geometries of the curved calibration standard reflecting surfaces, the resulting DAC plot obtained is much less accurate than that obtained when planar surfaces are involved.
The present day practice of using a straight calibration hole in a curved calibration standard thus exhibits three serious disadvantages. Firstly, the amplitude of ultrasonic waves reflected from the controlled geometry reflector surface is a function of transducer position with respect to the line of symmetry perpendicular to the calibration hole. Secondly, a continuous DAC plot cannot be obtained due to unavoidable focusing and defocusing of the beam caused by the curved calibration standard surfaces. Lastly, the location of the reflector surface within the calibration standard material cannot be accurately determined since the transducer to reflector distance varies across the width of the ultrasonic beam. Efforts to overcome these disadvantages in the past have not met with wide success.